Typical aircraft radar altimeters include a separate receiving antenna and transmission antennas located on the bottom of the fuselage of commercial or private aircraft. Separate transmit and receive antennas have historically been used in order to provide isolation between the transmitter and receiver during continuous transmission and reception of a radar signal. Transmitter to receiver isolation was required because of technology shortcomings of microwave signal sources and microwave device packaging technology. Similarly, microwave sources used in present radar altimeters used open loop methods because microwave devices did not exist to permit closed phase lock loops. Technology now readily available permits exceptionally low phase noise signals with exceptionally high quality linear frequency modulation under virtually any load or environmental conditions.
Operation of existing radar altimeters relies on a reflection of the transmitting antenna signal from the ground to the receiving antenna. At high altitudes, the separation distance between transmit and receive antennas results in a small reflection angle between the transmitted and received signals and provides excellent signal reception. At much lower altitudes as the aircraft lands, the reflection angle between the transmitting and receiving antennas becomes very large thereby attenuating signal reception at the outer reaches of the antenna beamwidths. Below a low altitude threshold the reflection angle will exceed the beamwidth of the transmitting or receiving antennas and altimeter operation will cease. Therefore, at low altitudes the separation distance between the two antennas of conventional radar altimeters reduces received signal strength compromising signal to noise ratio and altitude accuracy. At low altitudes, conventional dual antenna altimeters may erroneously acquire reflections from aircraft components such as engines and wheel gear instead of the correct ground reflection. A single antenna radar altimeter uses a single vertical reflection path to and from the ground not impacted by altitude or attitude of the aircraft. In special applications such as an aircraft tail-strike protection system there is a requirement to measure distances to the ground of less than one foot where a dual antenna altimeter will not function. Therefore, there are many needs for a single antenna FM radar altimeter.
The U.S. Pat. No. 6,426,717 to Maloratski presents a single antenna FM radar altimeter that performs continuous wave (FM/CW) modulation as well as an interrupted continuous wave modulation. FIG. 1 illustrates Maloratski's radar altimeter. Maloratski includes a circulator that directs transmission signals to the antenna or directs received signals through a radar-processing portion. Maloratski connects the circulator to the antenna via a coax cable, as it is the intent of the patent to remotely locate the radio frequency components of the altimeter from the antenna. Precision low range altimeter applications require exceptionally stable altitude data. However, temperature and moisture affect coax cables by increasing insertion loss, increasing reflection coefficients and changes in propagation delay time. Therefore, no means exists to continuously calibrate the true electrical length of the connecting cable. Any radar altimeter connected to its antenna or antennas via coax must calibrate propagation delay in order to know fixed distance to and from the transmitting and receiving antenna(s) caused by the electrical length of the coax for each aircraft installation.
Maloratski also presents closed loop analog circuitry for continuously adjusting modulation rate in order to produce a constant frequency received signal but the loop does not control the linearity or phase noise of the radar modulation. Any frequency modulated radar altimeter relies upon a nearly ideal linear modulation function of frequency change versus time. Maloratski's closed loop analog circuitry provides no means to verify that the modulation function is nearly ideally linear as a function of time, temperature or other environmental effects because it only controls the frequency of the received signal. In this way, Maloratski's approach uses an open loop modulation system.
Radio frequency sources of many types are subject to Frequency Pulling as a function of load impedance. As a result, open loop modulation systems suffer distortion in the linearity of the frequency modulation function due to the varying Voltage Standing Wave Ratio (VSWR) caused by coax cable deterioration or poor antenna matching. Poor modulation linearity results in degraded signal to noise ratio, altitude accuracy and causes errors in measurements of modulation rate.
Many conventional radar altimeters, including the single antenna altimeter proposed by Maloratski continuously adjust the period of the linear frequency modulation waveform as a function of altitude in order to achieve a constant received difference frequency. This constant received difference frequency is key to the altitude tracking mechanism of Maloratski and most prevalent radar altimeters. While this design feature provides a means to facilitate analog altitude tracking subsystems, it forces the altimeter to also provide an automatic gain control circuit that adjusts the amplitude of the received signal as a function of altitude and reflection brightness from the ground. This design feature complicates the altimeter design and imposes limitations to the response time of the overall altimeter circuitry with rapidly varying ground heights.
A basic concern for Frequency Modulated/Continuous Wave (FM/CW) radars with a single antenna is a large signal reflection from its antenna or connecting coax. Large amplitude reflections from the antenna or connecting coax cause the continuously transmitting radar to jam itself, thereby limiting sensitivity. Maloratski and others have utilized specialized cancellation circuitry in an attempt to prevent FM/CW self-jamming.
Therefore, present single antenna radar altimeter systems, like Maloratski, are overly complex, utilize open loop modulation and are relatively imprecise because of time and temperature changes and degraded RF performance due to coax cable degradation over time.
Therefore, there exists a need for a single antenna FM radar altimeter with no degradation in RF performance versus time, and no issues relating to connection distances between the antenna and the other radar altimeter hardware, and it is not prone to modulation errors, and is more accurately repeatable over time.